finding Concentration of Cr2O72-(aq) ?

Question

Problem

What is the concentration of a tin(ll) chloride solution prepared from a sample of tin ore?

Experimental Design

The potassium dichromate solution is first standardized by titration with 15.00 mL of an acidified 0.07 mol/L solution of the primary standard, iron(II) ammonium sulfate-6-water. The standardized dichromate solution is then titrated against a sample of the acidified tin(II) chloride solution (You will do this step in the next question). 

Evidence

	TITRATION OF IRON(lI) SOLUTION	(volume of K2Cr2O7(aq) required to react with 15.00 ml of 0.07 mol/L Fe2+(aq))
Trial	1	2	3	4
Final buretreading(ml)	13.8	24.4	35.2	45.9
Initial buretreading (ml)	2.3	13.8	24.4	35.2

Find the concentration of the Cr₂O₇^2-(aq) in mol/L: (give your answer to 4 decimal places)

Answer 1

Cr₂O₇^2- + 6 Fe²⁺ + 14H⁺ → 2Cr³⁺ + 6 Fe³⁺ + 7H₂O balanced redox equation

Find average volume of dichromate:

(1) 13.8 - 2.3 = 11.5 ml

(2) 24.4 - 13.8 = 10.6 ml

(3) 35.2 - 24.4 = 10.8 ml

(4) 45.9 - 35.2 = 10.7 ml

Mean volume = 10.9 ml = 0.0109 L

moles Fe²⁺ titrated = 0.01500 L x 0.07 mol/L = 0.00105 moles Fe²⁺

moles Cr₂O₇^2- required = 0.00105 mol Fe²⁺ x 1 mol Cr₂O₇^2-/6 mol Fe²⁺ = 0.000175 moles Cr₂O₇^2-

Concentration of Cr₂O₇^2- = 0.000175 moles/0.0109 L = 0.01606 M